The overall objective of this proposal is to gain further insight into the secretion and action of estradiol and to define the interaction of estrogen with LH and placental luteotropin in corpora lutea of pregnant rats. The first aim of this study is to determine the cellular origin of luteal estradiol synthesis. We will investigate the possibility that one luteal cell type contains LH receptors, LH-responsive adenylyl cyclase and cAMP-dependent protein kinase and responds to LH by secreting androgen, whereas the other luteal cell type aromatizes androgen to estradiol, contains estradiol receptors and produces progesterone when stimulated by estradiol. A long term luteal cell culture will be developed and the mechanism by which estrogen stimulates luteal production of progesterone will be investigated. Whether estradiol causes phosphorylation of cytosolic proteins, increases lipoprotein uptake, and affects cholesterol metabolism and enzymes activity in the steroidogenic pathway in luteal cells will be determined. Using enzyme inhibitors, we will determine whether the sustained action of LH on progesterone synthesis is mediated by its stimulatory effect on luteal testosterone. The possibility of an intracellular interaction between LH and estradiol will be investigated. We will specifically determine whether estradiol affects the coupling of LH receptor to adenylyl cyclase and modifies the state-of-activation or levels of cAMP-dependent protein kinase. We will then investigate why LH is not necessary for luteal cell function in the first seven days of pregnancy. Finally we will test the hypothesis that estradiol causes the demise of the corpus luteum at the end of pregnancy by stimulating luteal cell secretion of prostaglandins.